Oxygenation and mixing are required in many municipal and industrial wastewater treatment plants (WWTPs) to improve processing and shorten treatment durations. WWTPs often use mechanical aerators to both oxygenate and mix wastewater. However, much more aeration is needed to thoroughly mix than oxygenate the wastewater. Excessive horsepower (HP) usage for mixing increases electrical grid-power consumption, operational costs and greenhouse gas emissions. The New Hampshire Department of Environmental Services directed pilot studies at WWTPs in Exeter, Pittsfield and Rochester, New Hampshire, to investigate the ability of solar-powered circulation (SPC) to improve operational efficiency by oxygenating and mixing wastewater and reducing or eliminating the need for mechanical aeration while maintaining water quality. This report describes water quality parameters, sludge thickness, odor events, and kilowatt-hour (kWh) consumption and expenditures 1-year before- and 15 months during-SPC.

Biochemical oxygen demand (BOD), total suspended solids (TSS), dissolved oxygen (DO), ammonia and nitrate levels were systematically measured throughout the study period. Malodorous events, sludge and slurry thickness and electrical grid-power consumption and cost were also monitored. National Pollution Discharge Elimination System (NPDES) limits were met at each study site throughout the study period, and no odor events or significant sludge-thickness changes were observed. Exeter, a partial-mix pond system using aeration and six SPC units in the initial three lagoons totaling 28.1 ac, maintained DO levels between 0.5 and 1.8 mg/l in Lagoon 1, and > 2 mg/l in Lagoons 2 & 3 using only 87.5 of 327.5 HP aeration after month five. Electrical-grid kWh consumption declined by 45% while using SPC, resulting in an annualized cost savings of $89,160 and a 3-year payback period. Pittsfield, a partial-mix pond system using aeration and four SPC units in the initial four lagoons totaling 5.2 ac, used aeration as needed to maintain DO > 2 mg/l in all lagoons. Ammonia reductions at effluent of > 95% from June through early November indicated enhanced nitrification. Electrical-grid kWh consumption declined by 43% while using SPC, resulting in an annualized cost savings of $32,037 and a 3.9-year payback period. Rochester, an activated sludge system using five SPC units in a raw septage and a sludge-storage lagoon totaling 18.3 ac, used no mechanical aeration during the study period. Pre- versus during-SPC comparisons indicated slight increases in BOD and TSS, no change in ammonia and nitrate concentrations, and marked improvements in pH and alkalinity. Metals were not resuspended into the water column. Electrical-grid kWh consumption declined by 91% during the study period, resulting in an annualized cost savings of $127,449 and a 1.5-year payback period.

The pilot studies were considered successes; operational efficiency improved while water quality was maintained. Innovative mixing design powered by solar panels conserved DO and reduced electrical grid-power consumption and operational costs without greenhouse gas emissions.